In the course of analyses of the free amino acids in human blood plasma several quantitatively minor unidentified ninhydrin-positive materials have been observed consistently. The same materials are present in blood plasma from rats, rabbits and cows. It is proposed to identify some of these substances and to investigate their manner of origin and possible physiological significance. Deproteinized bovine blood plasma will be used as a source of the materials. Ion-exchange and thin-layer chromatography will be used for separation and purification of the compounds and the usual procedures of synthetic organic chemistry for the preparation of authentic materials.